Abstract
Trisomy 21 (Down Syndrome) is the model human phenotype for all genome gain-dosage imbalance situations, including microduplications. Years after the sequencing of chromosome 21, the discovery of functional genomics and the creation of multiple cellular and mouse models provided an unprecedented opportunity to demonstrate the molecular consequences of genome dosage imbalance. It was stated years ago that Down syndrome, caused by meiotic separation of chromosome 21 in humans, is associated with advanced maternal age, but defining and understanding other risk factors is insufficient. Commonly referred to as Down syndrome (DS) in humans, trisomy 21 is the most cited genetic cause of mental retardation. In about 95% of cases, the extra chromosome occurs as a result of meiotic non- nondisjunction (NDJ) or abnormal separation of chromosomes. In most of these cases the error occurs during maternal oogenesis, especially in meiosis I.